Gun tubes are rifled to impart spin to the projectile. The projectile is thereby stabilized and accuracy is enormously improved. Historically, smoothbore guns have given higher velocities, as the frictional drag between projectile and bore is reduced. Rifled barrels including smoothbore sections have been proposed from time to time and used successfully. The most dramatic example was the World War I Paris gun, a German development, which developed an astonishing range of 120 km by virtue of a muzzle velocity of 5260 fps. Seventeen meters of the tube was rifled; the last six meters was smoothbore at exactly the same diameter of the bottom surface of the rifling (i.e., groove diameter).
U.S. Pat. No. 3,525,172 teaches a method of duplicating the rifling form of the Paris gun, alleging that a smoothbore section of a diameter not less than the groove diameter following a rifled section improves velocity, especially when the transition from rifled to smooth barrel is placed at the point in the barrel where the peak pressure occurs (allegedly either 10.75 or 11.5 inches). However, in usual small arms design, the peak pressure occurs only a few bore diameters from the breech (approximately 0.75-1.5 inches).
In World War II, the Germans developed the tapered bore anti-tank gun, 7.5 cm 5.5 cm Pak 41. The bore tapered from 7.5 cm at the breech to 5.5 cm at the muzzle but the taper was not constant. The first part of the bore is cylindrical and rifled, the second, conical and unrifled and the third, measuring 27.6 inches in length, is cylindrical and unrifled. This gun utilized a Gerlich designed projectile that had a compressible outer case.
Another recent patent of interest, U.S. Pat. No. 4,126,955 for High Velocity Tapered Bore Gun and Ammunition, describes a gun barrel having a rifled section from which extends a smoothbore section tapering to a smaller diameter than the rifled section for reforming the projectile. With that structure, the projectile is reformed into a conical shape as it passes through the tapered section with, according to the '955 patent, beneficial results.
The combination of smooth and rifled barrels has been taught for years. U.S. Pat. No. 460,102 (1891) has a smooth section following the rifled section in a similar mode to the Paris gun and patents '955 and '172. Still another prior art patent, Australian Pat. No. 143,403, describes a rifled section followed by a larger diameter smoothbore section. This gun barrel functions as a dual purpose firearm to fire either bullets or cartridges loaded with shot. Russian Pat. No. 627,304 also discloses what appears to be an improvement over the Australian dual purpose gun barrel. It includes a rifled section followed by smoothbore sections which are used to expand the shot diametrically and cause it to lose some of the rotational moment it acquired in the rifled portion of the bore. Thus the smoothbore sections are provided to affect the shot, not the bullet.
It has been found that the prior art combination rifled and smoothbore guns have not significantly improved performance over conventional rifled barrels, and the designs have not been widely adopted or used.